As a manufacturing method of liquid crystal display cells, the so-called liquid crystal dropping process, which has a high mass productivity, is proposed accompanying the recent years' upsizing of liquid crystal display cells (see Patent Document 1 and Patent Document 2). Specifically, the manufacturing method involves dropping a liquid crystal inside a liquid crystal sealing material formed on one substrate, and thereafter laminating the other substrate thereon to seal the liquid crystal.
However, the liquid crystal dropping process, since a liquid crystal sealing material in an uncured state first contacts with a liquid crystal, has such a problem that components of the liquid crystal sealing material dissolve in the liquid crystal on contacting and the specific resistance value of the liquid crystal decreases, and as a result, such mass production method of the liquid crystal cell does not fully spread widely.
As curing methods of a liquid crystal sealing material after laminating in the liquid crystal dropping process, there are three methods of heat-curing, photocuring and combined photo- and heat-curing. The heat-curing method has such problems that a liquid crystal leaks from a liquid crystal sealing material rendered low in viscosity during curing due to the expansion of the liquid crystal by heating, and that components of the liquid crystal sealing material rendered low in viscosity dissolve in the liquid crystal. These problems are difficult to solve, so the heat-curing method is not yet in practical use.
On the other hand, liquid crystal sealing materials used in the photocuring method include two kinds of cationic polymerization type and radical polymerization type depending on the kinds of photopolymerization initiators. Since a liquid crystal sealing material of cationic polymerization type generates ions in photocuring, when it is used in the liquid crystal dropping process, it has such a problem that the ion components elute in the liquid crystal in the contact state and the specific resistance value of the liquid crystal is reduced. Then, since a liquid crystal sealing material of radical polymerization type has a large curing contraction on photocuring, it has a problem of an insufficient adhesion strength. A problematic point related to both photocuring methods of cationic polymerization type and radical polymerization type involve such a problem that metal wiring parts of an array substrate for liquid crystal display cell and black matrix parts of a color filter substrate generate light-shielded parts of a liquid crystal sealing material where light does not enter, thus causing the light-shielded parts to become uncured.
The heat-curing method and the photocuring method have such various problems, and actually the combined photo- and heat-curing method is believed to be the most practical method. The combined photo- and heat-curing method is characterized in that a liquid crystal sealing material interposed between substrates is irradiated with light to be primarily cured, and is thereafter heated to be secondarily cured. The important characteristics required for a liquid crystal sealing material used in the combined photo- and heat-curing method involve that the liquid crystal sealing material does not contaminate a liquid crystal in every process before and after light irradiation and before and after heat-curing, and especially coutermeasures for the light-shielded parts described before, namely, countermeasures against the elution of the sealing material components from the parts of the sealing material which have not be photocured into the liquid crystal on heat-curing are required. As solving methods, the following are conceived; (1) curing rapidly at a low-temperature before sealing material components elute out, (2) constituting components of a sealing material which hardly elute in a liquid crystal composition, and the like. Of course, the rapid curing at low-temperature accompanies worsening of the pot life during use, which is a practically big problem. Therefore, to obtain a liquid crystal sealing material which has a long pot life and exhibits a low contamination of a liquid crystal, the liquid crystal sealing material is required to be constituted of components which hardly elutes in a liquid crystal composition.
However, since a commonly well known epoxy resin such as bisphenol A epoxy resin and bisphenol F epoxy resin has a good compatibility with the liquid crystal, it cannot be said that said epoxy resin is suitable as the sealing material constitutional component in view of contamination.
In Patent Document 3, use of a partially (meth)acrylated bisphenol A epoxy resin described in Patent Document 4 as the main component of the resin is proposed as the liquid crystal sealing material for dropping process (refer to Patent Document 3 and Patent Document 4). However, although solubility to the liquid crystal is reduced by (meth)acrylation, it cannot be said to be sufficient, and further it is difficult to solve the problem that the liquid crystal is contaminated with the unreacted remaining raw material epoxy resin.
As described above, a combined photo- and heat-curable liquid crystal sealing materials for liquid crystal dropping process, which has been conventionally proposed, cannot be satisfactory in view of contamination into a liquid crystal, adhesive strength, available time for use at room temperature and curability at low temperature.
Patent Document 1: Japanese Patent Application Laying Open (KOKAI) No. 63-179323
Patent Document 2: Japanese Patent Application Laying Open (KOKAI) No. 10-239694
Patent Document 3: Japanese Patent Application Laying Open (KOKAI) No. 2001-133794
Patent Document 4: Japanese Patent Application Laying Open (KOKAI) No. 5-295087